1. Field of the Invention
This invention pertains to sound projection devices. More particularly, the present invention relates to a device and method for enhancing a directional parametric speaker while reducing the quantity of ultrasonic emitters required.
2. State of the Art
Recent developments have been made involving sound propagation from parametric speakers, acoustic heterodyning, and other forms of modulation of multiple ultrasonic frequencies to generate a new frequency. In theory, sound is developed by the interaction in air (as a nonlinear medium) of two ultrasonic frequencies whose difference in value falls within the audio range. The resulting compression waves are projected within the air as a nonlinear medium.
An interesting property of parametric sound generation is enhanced directionality. Despite significant publications on ideal theory, however, general production of sound for practical applications has alluded the industry for over 100 years. Specifically, a basic parametric or heterodyne speaker has not been developed which can be applied in general applications in a manner such as conventional speaker systems.
A brief explanation of the theoretical parametric speaker array is provided in xe2x80x9cAudio spotlight: An application of nonlinear interaction of sound waves to a new type of loudspeaker designxe2x80x9d by Yoneyama et al as published in the Journal of Acoustic Society of America, 73(5), May 1983. Although technical components and the theory of sound generation from a difference signal between two interfering ultrasonic frequencies is described, the practical realization of a commercial sound system was apparently unsuccessful. Note that this weakness in the prior art remains despite the assembly of a parametric speaker array consisting of as many as 547 piezoelectric transducers yielding a speaker diameter of 40-50 cm. Virtually all prior research in the field of parametric sound has been based on the use of conventional ultrasonic transducers, typically of bimorph character.
A common structural feature of prior art attempts to develop an effective parametric speaker is to form a substantially continuous array of transducers across the surface of a support plate. The natural assumption appears to be that filling in the interior area of the support plate with the maximum number of transducers is appropriate to maximize sound pressure level (SPL). Conventional speaker theory would suggest that increasing the number of transducers would in deed contribute to increased SPL. Accordingly, prior art parametric speakers are typically illustrated with bimorf transducers compactly positioned in honeycomb array.
Although such parametric speakers have created some interest, it has seemingly been restricted to scientific curiosity. The development of practical applications and products has been very limited. The efficiency of such systems has apparently not been adequate to suggest utility in applications as part of a commercial audio speaker system.
It is an object of the present invention to provide a method and apparatus for indirectly emitting new sonic and subsonic waves with less power requirements than with prior art parametric speakers.
It is another object to structurally adapt a parametric speaker to produce a narrow beam of new sonic or subsonic energy with less distortion and using less emitter surface than previously experienced with parametric speakers of comparable beam diameter.
A further object of this invention is to increase efficiency of a parametric system by significant reduction in emitter surface area without a corresponding proportional reduction in SPL.
These and other objects are realized in a parametric speaker device which comprises a support base having a sonic or subsonic emitting perimeter positioned around a central open section, wherein the open section has a diagonal width greater than a cross-sectional diagonal of the emitting perimeter of the support base. The device may also include circuitry components such as an ultrasonic frequency signal source for generating a first ultrasonic signal, a sonic or subsonic frequency generator for supplying an electrical signal corresponding to the at least one new sonic or subsonic frequency, and a modulating circuit coupled to the ultrasonic frequency signal source and sonic or subsonic frequency generator for mixing the first ultrasonic frequency signal with the electrical signal corresponding to the at least one new sonic frequency to thereby generate a waveform including the first ultrasonic frequency signal and a second ultrasonic frequency signal. The emitting perimeter comprises ultrasonic frequency emitting material which can be coupled to an output of the modulating means for (i) propagating a waveform embodying both the first and second ultrasonic frequency signals, and (ii) generating the at least one new sonic frequency as a by-product of interaction between the first and second ultrasonic frequency signals.
The invention is also represented by a method for enhancing efficiency of a parametric speaker system with respect to energy output based upon emitter surface area, comprising the steps of a) forming an ultrasonic frequency emitting perimeter on a support base around an open region which is substantially void of ultrasonic emitting material; and b) emitting ultrasonic frequency from the emitting perimeter to generate sonic or subsonic sound within surrounding air as part of a parametric speaker system.